As is well known in the art, nickel-copper alloys have been used in sundry, diverse applications for decades. A number of such materials contain sulfur, a constituent often deemed subversive depending upon the intended application. However, recourse is found in the use of sulfur where excellent machinability characteristics are required, an operation which can be otherwise markedly expensive if difficult to perform. As a consequence, there are specifications, including specifications of Defense Procurement Agencies, e.g., QQ-N-281 (Class B), which require the presence of minimum sulfur levels, e.g., 0.025%.
As above indicated, sulfur can detract from properties since in nickel-copper alloys it is causative of forming nickel sulfide, an embrittling phase. To offset this, magnesium is used to transform the sulfur from an embrittling NiS grain boundary film into a less harmful globular magnesium sulfide (MgS) precipitate. However, as beneficial as magnesium may be, MgS tends to excessively plastic at conventional hot working temperatures, say 1800.degree. F. to 2200.degree. F. Apparently, the magnesium sulfide results in the formation of what are termed in the art as elongated "stringers". Put another way, upon hot working these elongated stringers form as opposed to discrete particles, i.e., the hot rolling does not sufficiently break up the stringers into particles. By reason of this, during hot working and/or upon subsequent cold working, undesirable cracking ensues leading to "material rejects".
The problem has manifested itself, for example, in the production of fasteners such as hexagonal nuts. In the production of such end products alloy material is upset forged, sliced to short lengths, hole punched and then threaded. In forming the hexagonal nuts troublesome cracking has been encountered on the outside diameter of the nuts leading to excessive rejects which, in turn, lead to unnecessarily higher production costs.
Sulfur removal might prove a panacea and this could possibly be accomplished by, inter alia, flux smelting. But the presence of sulfur is necessary for machinability applications. Accordingly, the problem was one of retaining the benefits of sulfur in respect of the machinability of nickel-copper alloys while minimizing the "stringers" adverse affect such that the alloys could be both hot and cold worked without excessive cracking.